Conventional antifouling films known to have antifouling properties include a coating film shown in Patent Literature 1. The coating film will now be described with reference to FIG. 7. In the following description, the term “attached” is defined as a state where contaminants such as dust are in contact with a surface. The term “adhered” is defined as a state where contaminants are stuck to the surface. The term “antifouling” is defined as the ability to prevent contaminants from being attracted, attached, or adhered.
FIG. 7 is a configuration of a coating film as the conventional antifouling film. In FIG. 7, coating film 101 is coated on object 106 to be coated, which is made of plastic such as polystyrene or polypropylene. Coating film 101 is composed of silica film 102 having a smooth surface and containing silica microparticles 104; and fluororesin particles 103, which are half buried and half exposed on silica film 102. Silica microparticles 104 of silica film 102 have a particle size of 4 nm to 15 nm, whereas fluororesin particles 103 have a particle size of 50 nm to 500 nm. Hydrophilic silica film 102 conducts some electricity, thereby providing an antistatic effect to dissipate electric charges accumulated on its surface. Hence, silica film 102 can prevent dust from being attracted and attached to its surface due to electric charges.
Silica film 102 can prevent dust attachment also because it has a smooth surface with no projections-and-recesses in which dust may be caught. In addition, coating film 101 has fluororesin particles 103 sticking out of its surface, so that the surface area of silica film 102 is not large enough for hydrophilic dust such as sand to adhere. Fluororesin particles 103 are composed of fluorine atoms and carbon atoms bonded into molecules. The bonding orbitals of the fluorine atoms are filled with paired electrons. Hence, the fluorine atoms do not combine with other substances, allowing fluororesin particles 103 to have an inert surface. This prevents adhesion of not only hydrophilic dust such as sand, but also hydrophobic dust such as carbon and soot. Furthermore, coating film 101 contains oxidizing agent 105, which functions to improve the intimate contact between coating film 101 and object 106 to be coated.
Coating film 101 shown in Patent Literature 1 has projections and recesses intentionally formed by making fluororesin particles 103 stick out of the smooth surface. Coating film 101 with a smooth surface has a surface area large enough for dust to be attached. To obtain higher antifouling properties, it is necessary to minimize the surface area to which dust may be attached, and to make attached dust afloat.
The dust-attracting action can be reduced by quickly dissipating electric charges from coating film 101, but the problem is that its conductivity is not high enough for that.
The intimate contact between coating film 101 and object 106 to be coated is improved by the function of the oxidizing agent. When washed with water, however, coating film 101 is dissolved in water and disappears because silica microparticles 104 of coating film 101 are hydrophilic. This causes a reduction in the antifouling properties until they are finally lost.